The present invention relates to a device for detecting the intensity distribution of X-rays over a two-dimensional area. More particularly, the invention relates to such a device comprising an X-ray converter foil with a semiconductor layer and a carrier layer.
In X-ray diagnostics, the X-ray detection and recording system must be capable of making visible to the radiologist the biological structures relevant to the diagnostic process. Because even a very small amount of radiation can damage an organism, the radiation exposure of the patient must be minimized as much as possible. Improved picture quality, however, generally requires increased radiation exposure. In addition to maximizing detection while minimizing exposure, the X-ray recording system must also be suitable for clinical routine operation, i.e., it should be easy to handle and reliable.
An X-ray image which contains information about an irradiated object is made visible for the human eye generally by means of a device which converts incoming X-rays into a two-dimensional image, e.g., by the exposure of a film and/or the generation of fluorescent radiation. Currently, the direct exposure of X-ray sensitive film by X-rays is used in only a few examinations in medical radiology, since the lower quantum yield of the film requires a high radiation exposure of the patient. More frequently, the X-ray pictures are made by means of a sandwich of two so-called amplifier foils with X-ray film placed therebetween, the use of this foil sandwich being known as the screen-film technique. In this technique, the film is exposed mostly indirectly via fluorescent light emitted by the amplifier foils. The amplifier foils increase the quantum yield due to their higher density and the heavy atoms contained therein.
An X-ray recording system designated the XEROX-125-System uses a carrier plate or foil consisting of aluminum on which is vapor-deposited the semiconductor selenium. For sensitivity reasons, the selenium layer frequently has a thickness of at least 135 .mu.m. This X-ray converter foil is charged by a positively charged wire which is stretched parallel to the surface of the selenium layer and at which a corona discharge burns, the charging process occurring in the absence of electromagnetic radiation including optical, X-ray and ultraviolet frequencies. The selenium layer is charged to a uniformly positive potential of, for example, 1,000 V. The X-ray converter foil is placed in a cassette for exposure to X-rays arriving in a two dimensional intensity distribution. The rays passing through the object induce photoconductivity in the selenium, whereby part of the surface charge on the selenium layer can flow to the aluminum carrier plate. A latent electrostatic image remains at the surface of the selenium layer, which electrostatic image corresponds to the X-ray image. (W. Seelentag, "Elektroradiografie," Physik in unserer Zeit, Vol. 10, 1979, No. 4, pages 101 to 113).
The latent electrostatic image can be made visible on paper with a dry powder or by a powder and liquid development process, such as that used in conventional photo copiers, or can be detected by means of a potential probe and reproduced on a monitor. Because the surface of the X-ray converter plate is generally charged up to a potential of several thousand volts, the diameter of the potential probe and the distance of the potential probe from the surface of the X-ray converter plate have predetermined minimum values which are determied in part by the dielectric strength of air. In addition, the quantum yield of an X-ray converter foil having a layer of selenium on an aluminum carrier plate is small and requires a relatively high radiation exposure.
An object of the invention to provide an X-ray converter foil having a substantially increased quantum yield whereby the radiation exposure of the patient can be decreased and whereby the loss of local resolution in the transmission or the read-out of the latent electrostatic image is reduced.